Variable amplifying device



l 1941- L. E. RYALL 2,255,190

VARIABLE AMPLJFYI NG DEVICE Filed Jan. 27, 1940 2 Sheets-Sheet l Fig. 1

\NVENTOR L.E.RYALL Sept. 9, 1941. 1.. E. RYALL.

VARIABLE AMPLIFYING DEVICE Filed Jan. 27, 1940 2 Sheets-Sheet 2 INVENTORL..E.RYA1.\J

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Patented Sept. 9, 1941 VARIABLE AMPLIFYING DEVICE Leonard Ernest Ryall,Pinner, England Application January 27, 1940, Serial No. 316,015 InGreat Britain February 18, 1939 11 Claims.

The present invention concerns improvements in or relating to amplifyingcircuits and has for its object the provision of a circuit whichissuitable for use as an amplfier in which the gain is controlled inaccordance with the signal input thereto and has the further object ofproviding a circuit which is of particular advantage in connection withthe reception of alternating current signals.

According to one feature of the present invention current is fed backfrom the output to the input circuit of an amplifying device so as tocause the device to be normally in a state of oscillation, said feedbackpath including a nonlinear impedance device by which the amount offeedback is varied with the strength of the signals received in such amanner that the amplification of weak signals is greater than theamplification of strong signals. According to a further feature of theinvention the gain or amplification factor of an amplifying device iscaused to vary with the strength of the signals received by providing afeedback path between the output and the input to bring the device intoa state of oscillation, which includes an impedance device variablycontrolled by the output to modify the amount of feedback so that theamplification of the stronger signals is less than the amplification ofthe weaker signals.

In receiving alternating current signals of a predetermined frequencyfrom a circuit over which currents of other frequencies are allowed topass it has been usual to provide a limiting device so that the actualreceiving equipment tends to receive current of constant amplitude orapproximately constant amplitude. Such devices in practice are found tohave the disadvantage that they introduce harmonics of the fundamentalfrequency of the signal which is disadvantageous, particularly in thecase where currents of other frequencies than the signalling frequencyare used as these harmonics have to be rendered more effective as theywould otherwise tend to weaken the sensitivity of the device forreceiving the signals of predetermined frequency and according to afurther feature of the invention such limitation is brought about by animpedance device having a time factor such as a resistance in vacuum orin an inert gas which causes the amplitudes of the weaker and strongersignals to more nearly approach each other without causing thegeneration of harmonic frequencies before reaching the tuningarrangement by which currents of the predetermined frequency are aloneenabled to control the signal responding device, the time factor beingso chosen that the device will not respond to variations correspondingto the harmonic component of the signal but only to changes which takeplace over a period greater than one cycle of the predeterminedfrequency. That is to say, the device does not respond to theinstantaneous values of the currents received but rather to the mean orR. M. S. value of such currents.

The preferred form of device for. meeting the above conditionsconveniently takes the form of a filament wire in a vacuum or gas-filledenvelope whose resistance varies in accordance with the current flowingthrough it having regard to the fact that due to the heat inertia itwill not respond to very rapid changes of current. Pref erably theadjustment of the feedback path of the thermionic valve amplifier deviceis arranged so that the valve device is in the condition of 0S-cillation and the effect of a received signal is to cause the system tostop oscillating. As is well known in this condition the amplificationfactor of the valve is a maximum and the greaterthe amplitude of thereceived signal the greater will be the departure from the point ofoscillation and consequently the less will be the amplification of thesystem.

One arrangement for carrying the invention into effect will be describedby way of example with reference to the accompanying drawings whichillustrate the application of the device to the reception of signallingcurrents of two different predetermined frequencies. It will beunderstood however that the invention can be readily applied to thereception of a single predetermined frequency and also for'otherpurposes in which pentode type, although other forms of valves.

are suitable, or combinations of more than one valve may be employed.The input for valve I0 is fed in over. the terminals ll. [2 which may beconnected to a telephone line or cable or to a wireless receiver or thelike. The current passes via the transformer l3 to the gridcathode-circuit which includes a high impedance I4 and decouplingcondenser A. The input circuit also inciudes the secondary winding of atransformer IS, the primary of which is connected to the output circuitas will be described later. The primary of the input transformer I3 isconnected to the line through two resistors of such a value that thetapping ios is of such a value that for a voice frequency range from 200to 3000 cycles per second, the tapping loss is kept within very lowlimits. If the transformer I5 is disconnected the amplification of thevalve i is reasonably uniform over the voice frequency range. The outputcircuit of the valve l0 includes the primary winding of a transformer l6and a decoupling resistance l1 and condenser HA. The

lower secondary winding of the transformer [6 has a central tapping andis arranged to form two arms of a bridge arrangement, the other two armsof which consist of a fixed resistance l8 and a non-linear resistanceIS, the primary of the transformer l being connected to the midpoint ofthe transformer winding and to the Junction point of the resistances l8and IS. The upper secondary winding of the transformer 16 is connectedto the grid and cathode of a valve 20 the output of which includes theprimary winding of a transformer 2| having three secondaries. Theuppermost secondary serves to supply a tuning device for one of thepredetermined frequencies, say 750 cycles and comprises a seriescondenser 22 and a shunt inductance 23, across the inductance being arectifier bridge 24. from which currents are fed to the input circuit ofthe valve 25'having a responding relay 26 (preferably quick acting) orits equivalent in the anode circuit. The circuit connected to the middlesecondary winding is similar except that the rectifier bridge isconnected across the condenser and it is tuned to a different frequency,for instance 600 cycles per second and controls via a valve 21 a relay28. The circuit connected to the lower winding constitutes a guardcircuit which is adapted to reject currents of frequencies to which thetwo above-mentioned devices respond by means of the rejector circuits 29and 30 and its sole purpose is to provide a potential in the gridcircuit in opposition to the potentials generated in the grid circuitsof the valves 25 and 21 so as to render these valves non-responsive orless responsive. The latter arrangement acts as a guard circuit and needonly operate on frequencies away fromthe signal frequencies, for

instance above 1000 cycles per second and below so effective in practicethat when used in conjunction with compound signals wide band signalacceptor circuits can be used and the tuned circuits need only have lowQ values. An effective band width of 300 cycles per second can beprovided for impulsing so that almost rectilinear impulse signals areobtained. If the rectified guard voltage has a longer build-up time anda longer decay time than the rectified signal voltage the wave front(and end) of the signal impulse is not affected by the guard voltage asthe front (and end) have formed before the effect of the guard voltagecomes into play. Hence a larger guard voltage could be used withoutaffecting the impulse characteristics. The equivalent band width used inimpulsing is derived from the rectified signal only which is larger thanwhen the guard voltage is taken into account. Such a receiver may bedesigned so that no adjustments need be made when installed and theimpulsing distortion is very small even when supply voltages and valvesvary over wide limits.

The signal from the line or other source is passed via a high impedanceinput circuit to the oscillating amplifier I 0 where the output power iscontrolled so as to be substantially constant; for instance with aninput variation of 25 db. the output may be constant to within +or -1db, The actual value of the output is solely determined by the wireresistor I! which varies with signal strength in conjunction with theresistor i8 which does not vary. The constant output is fed to a bufferamplifier stage 20 provided with negative feedback to makethe voltageindependent of load, valve and supply variations.

The valves 25 and 21 are normally biassed so that the anode current isalmost zero and the rectified signal voltages applied to their gridcircuits cause the anode currents to increase and operate relays 28 and20 which are conveniently of highspeed type.

The signal fed back through the anode circuit to the grid circuit ofvalve l0 it will be noted is taken from the diagonally opposite cornersof a bridge circuit and the automatic volume control device of theinvention operates by changing the feedback from positive to negative asthe input level increases. The non-reactive resistor l8 hasapproximately twice the impedance of the resistor IS with no currentflowing. The resistor l9 conveniently consists of a metal filament lampand has a voltage current characteristic and a voltage resistancecharacteristic of the kind shown in Fig. 2. The bridge is thereforeunbalanced so that a considerable positive feedback effect is normallyobtained which is arranged to be sufllcient to produce oscillation. Theoscillation currents heats the lamp filament I! so that its resistanceincreases and reduces the feedback. This continues so that oscillationis only Just maintained. Under this condition the resistance of resistorI9 is only very slightly less than resistor I8 and a very small changeof the current in i9 is then sufficient to reduce the positive feedbackto zero or even to change it to negative feedback whenever theresistance of i9 exceeds that of I8. The frequency responsecharacteristic of the circuit when not in oscillation should be nearlyuniform over the range of frequencies over which it is desired tooperate the amplifier. The oscillation frequency occurs at the frequencyof maximum amplification and this is arranged to be just outside theoperating frequency bands of the signals to be received. When an inputof signal or normal speech frequencies is received via the transformerII the signal is initially amplified considerably, since a system inoscillation has an infinite amplification at the oscillating frequencyand a very high amplification at other frequencies. The increased outputcauses the temperature of. the lamp to rise-and consequently theresistance of I! to increase. This causes the quiescent oscillation tostop and reduces the positive feedback with the result that theamplification of the system will fall until equilibrium is established.The output from the amplifier-now corresponds in frequency to the inputinto transformer II and the RMS amplitude is very slightly above that ofthe quiescent oscillation since the lamp resistance I9 is only changedto a very small degree. Any further increase of input to transformer Itcauses only a very slight increase in output which is sufficient toaffect the feedback so that the amplification is reduced by an amountalmost correspondingto the increase in signal input level.

Advantages of this entirely by simple resistance values of and I9 andtheir resistance temperature coefficients. These values can bedetermined very simply and do not change with time so that externalinfluences cannot affect the operation appreciably. Variations of valveamplification due to changes in the amplifying valve and the anode andheater.

supplies only change the level at which the input signal takes controlto cause the natural oscillations to cease. By using a pure metal suchas copper for the resistance, I8' having the same resistance coeflicientas I! (approximately .004 ohm per degree centigrade for tungsten) theoutput level becomes practically independent of temperature variations.

A further advantage of the invention is that owing to positive feedbackthe circuit provided a large maximum amplification.

A further advantage is that the quiescent oscillation maintains thetemperature of the lamp is at the operating value so that very littleextra heat has to be introduced when a signal is received in order toproduce the desired volume control. Hence the speed of operation is highwhile a further advantage is that negligible harmonic distortion isintroduced. The RMS output is maintained constant instead of as withother automatic volume control circuits which operate on peak voltagesand in which the level of each component of the two-frequency signal maybe reduced as much as 6 decibels as compared with the level of a simplesignal assuming that the signals are each of the same amplitude. Byoperating on RMS voltages the reduction in amplitude is much less.

The valve l may take the form of any valve with a good mutualconductance, for instance an electron multiplier could be used. Thelarger the amplification of the valve however the smaller is the amountof feedback required to maintain oscillation and hence the more nearlythe resistance of I! can approachthe resistance of It so that very smallchanges of the resistance of IQ can effect a large degree of feedbackcontrol. The output of the valve is limited by the decoupling resistor Hin the anode circuit so that when operating with, for instance. 150 voltanode supply, the maximum power obtainable is only just sufficient tocause the lamp I! to glow a dull red. The lamp cannot therefore be burntout by excessive input voltages. The resistance lamp is can also be ofstandard design, for instance, a lamp having a nominal rating of 6 volts40 milliamps has been found to be very satisfactory. The lamp normallyoperates with arrangement are that the controlled output level can bedetermined almost a resistance of about 40 ohms when the voltage acrossit is .33 volt, so that is less than 3 mw.

its power consumption as compared with its lamp .rating of 240 mw. Thusthe lamp acts solely as a resistance wire operating in a vacuum and hasan indefinitely long and constant life. The equal windings feeding thebridge of the lower secondary winding of the transformer ii aretwinderived from the inductance component wound for balance. Theresistor l8 should be of such dimensions that the signal no appreciablerise in temperature and may conveniently be 40 ohms, non-inductivelywound with copper wire so cient is equal to that of the tungsten wireused in the lamp. If the resistor IB' is inductive, oscillation mayoccur at a very high frequency. The arrangement is such that bothfrequencies of signals to be received are amplified to the same degreewhen a compound signal is received so that equal proportions of the twosignals should be present in the final amplitude-controlled signal. 1

Speech si als of frequencies above and below the predetermined signalfrequencies which are to provide a guarding function to prevent thesignal receivers operating by speech receive the same amplification atthe predetermined frequencies so that an effective guard voltage isderived from the speech. The feedback transformer It also has areasonably uniform response characteristic but its maximum response isobtained at the frequency at which the quiescent oscillation occurs, forinstance 340 cycles per second. The oscillation frequency is controlledby the capacitor ll and resistor 3|A and must be outside the range offrequencies that operate the receiver.

In order to obtain fiat response characteristics for the transformers i3and II in conjunction with high stepup ratios, "MumetaY cores-are to thecapacity to earth associated with the secondary winding of thetransformer I3. The level of this oscillation is however very small soas to be substantially negligible, The intermediate amplifier stagerepresented by valve 20 is provided to enable adequate power to beapplied to the frequency selecting circuits. without afiecting thefrequency response characteristic of the oscillating amplifier circuit.Negative feedback is incorporated by coupling the anode to the grid biasend of the transformer winding associated with the grid of this valve.Approximately sufficient feedback is obtained to make the output voltagefrom the valve independent of the valve characteristics and of the loadimpedance. A large value of the cathode bias resistor 32 is used toreduce the anode current drain. The impulsing characteristics of thereceiver are not critically dependent on the signal level and the effectof the changing output on voice immunity is very slight since the guardcircuit voltage changes in the same ratio as that applied to the signaloperating circuits.

The higher frequency signal to 'be rectified is of the series tunedcircuit 22, 23 which is connected across one winding of the outputtransformer 2|. By deriving the signal from across the inductanceinstead of across the capacitor, a response below the signallingfrequency falls off rapidly so that the lower frequency signal cannotoperate the higher frequency signal relay while at the same time a largeupper side band is obtained which is required for good impulsing. In theI current causes that its temperature coefli be rectified is derivedfrom across the capacitor component of the series tuned circuit so thatthe response above the lower frequency falls of! rapidly. The guardcircuit which consists of parallel tunedcircuits to reject both thesignalling frequencies, is connected in series with a resistance 33across which the output voltage is obtained. The impedances of the twoparallel tuned circuits are approximately equal at their respectiveresonance frequencies. It is realised that a guard circuit operatingfrom a larger input voltage would be more effective in providing speechimmunity but it may reduce the available side band for 'impulsing ifsuitable precautions are not taken such as making the guard circuit withlonger build up time and longer decay time as described above. Since thetransmitted side bands contain both very low and very high frequencycomponents guard circuit should be too sensitive even at frequenciesdistant from the impulsing frequency. The quiescent oscillation which ispresent immediately Prior to the impulsing signal being receivedprovides a negative grid bias voltage which however has practically noeffect on the impulse ratio. The time constants of the rectifiercirefiicient conditions.

The charging time of the voltage-doubler capacitors depends on thevalues of the capacitor capacitance and the forward impedance of therectifier which is a function of the voltage across its terminals. Ifonly a small rectified voltage were required to operate the receiverrelay the voltage across the rectifier would be an appreciable amount ofthe total voltage available. forward impedance of the rectifier wouldthen materially affect the rectified voltage wave-form. Actually the useof rectifiers having five times the forward impedance of the normalrectifiers for the same potential diflerence across the rectifierchanges the impulse ratio by less than 1%. The time constant of thedischarge circuit apart from being very short is practically unaffectedby the backward impedance of the rectifiers.

The guard circuit voltage is voltages are applied to the control gridsof the signalling valves through high resistances 34, 35. Theseresistors limit the grid current which can be obtained when a largerectified voltage from either of the signalling frequencies is obtained.Otherwise the capacitors in the guard circuit rectifier system would becharged by the grid current derived from the larger signal and an excessnegative bias voltage obtained which may 'prevent operation of the relayassociated with it is undesirable that the age to operate both thesignal relays. The response characteristic of" theguard -circuit. playsand important part in achieving this result. Without the guard circuitit would not be possible to obtain a wide effective frequency band usinglow Q tuned circuits without operating both relays by means of a singleintermediate frequency signal. The higher frequency signal responsecharacteristic conveniently extends from 600 to 900 cycles per second sothat a wide side band is available. This side band is attenuated at theremote side-band frequencies and a side-band tangular signals. The widefrequency sid band is possible mainly because voice immunity is signalproduces a larger guard voltage than with the single frequency actingalone due to the larger contribution to the guard signal of the providesconsiderable negative variations in the valve charnegligible effect onthe perfeedback so that acteristics have a formance.

that derived from the valve 20 which serves a similar function for thevalve 25. To compensate for the high side-band attenuation of the higherfrequency in an approximate manner a two microfarad capacitor 38 isconnected across the resistor 36. The effect of the capacitor is to makethe impulse more rectilinear and the distortion is consequently reduced.Similar remarks apply to the capacitor 39 across the resistor 37.

By using a very sensitive high speed relay for the relays 26 and 28 itis possible to arrange that signals of one quarter the amplitudenormally received are sufficient to operate the receiver and therebyvery large factors of safety can be introduced. Capacitor 40 by-passesthe doublefrequency signal component in the anode circuit of the higherfrequency signalling valve 25. This impulse ratio using different relaysis very small. The arrangement of relay and by-pass capacitors for thelower frequency valve is similar to those used for the higher frequencyvalve. By the arrangement 9. very high degree of voice immunity isfrequency range to prevent operation of one or both of the relays. Aftermuch practice a certain voice intoning a quiet note of the requiredcharacter has succeeded in operating the receiver. When no delay timeswhatsoever, have been introduced one or both of the relays in thesecircumstances is in a chattering condition and it is believed thatoperation can only be obtained on transient signals, that is signals ofwhich the frequency spectrum is continuously changing. By introducing100 milliseconds delay before operation of the receiver effectiveoperation becomes impossible by voice sounds in a telephone transmitter.

As a modification to the arrangement above described the valve Ill maybe arranged to oscillate over a portion or portions of the frequencyrange and to be in a non-oscillating condition over another portion orportions of the range. For example when applied as a current limitingdevice for voice frequency signals such as are at present employed overtelephone lines for signalling, dialling and other-operations, theoutput transformer l6 may be replaced by two transformers having theirprimary windings in series. The secondary winding of one transformer isarranged as described above to give the necessary feedback. The othertransformer has connected in series across-the secondary winding thereoftwo parallel tuned circuits of frequencies equal to the frequenciesemployed for signalling and a third untunedimpedance. The impedance ofthis last transformer is a maximum at the two frequencies in question sothat at those frequencies but small proportion of the output voltageappears across the primary winding of that transformer from which thefeedback voltage is derived. The signal currents are derived from thevoltages across the two tuned circuits and a guard circuit may beoperated from the aperiodic impedance. In place of the two transformersone having four secondary windings may be employed using series-tunedinstead of parallel-tuned resonance circuits. The slope of the appliedvoltage impedance characteristic of the impedance device I9 exhibitsvarying slopes over varying portions of the characteristic and it isadvantageous to pass polarising current through the impedance to adjustthe operating point on the characteristic to the position of maximumslope. Where the thermionic valve is operated in an oscillatingcondition the oscillating current itself may be adjusted to give theoptimum operation.

I claim:

1. An amplifying circuit for receiving alternating current signals,comprising an amplifying device having an input and an output circuit, apath from said output circuit to said input circuit including animpedance device variably controlled by current in the output circuit,and a device tuned to a frequency outside the range of frequencies to bereceived, the arrangement of said impedance device and said tunedcircuit being such that when no signals are being re-v ceived theamplifying device oscillates at a frequency determined by said tuneddevice and that when signals are being received the current fed,-

5 I quency outside the range offrequencies to be received, thearrangement of said non-linear impedance device and said tuned devicebeing such that when no signals are being received the amplifying deviceoscillates at a frequencyv determined by said timed device and that whensignals are being received the current fed back over said path causesthe oscillations to cease and the amplification of the stronger signalsto be reduced as compared with the amplification of the weaker signals.

3. A receiving circuit for alternating current signals of apredetermined frequency comprising an input circuit, a signal respondingdevice, a tuning arrangement between said input circuit and said signalresponding device by which currents of predetermined frequency are aloneenabled to reach said signal responding device, a variable impedancedevice located between said input circuit and said tuning arrangementand having a time factor which causesthe amplitudes of the weaker andstronger signals to more nearly approach each other without causing theappreciable generation of harmonic frequencies.

4. A receiving circuit for alternating current signals of apredetermined frequency which may be mixed with currents of otherfrequencies, comprising an input circuit, a signal responding deviceresponsive only to signals of predetermined frequency, adeviceresponsive to other frequencies arranged to reduce the sensitivity ofsaid signal responding device and a device for reducing the variation ofamplitude of all currents received over said input terminals'whichoperates relatively slowly compared with the signal frequency and soproduces negligible signal distortion and harmonic frequencies so thatthe sensitive reducing device is not affected substantially by theoperation of said amplitude variation reducing device.

5. An amplifying circuit for receiving alter output circuit to saidinput circuit, said path including an impedance device whose impedancechanges in accordance with the R. M. S. value of current flowing throughit, whereby a substantially constant output is obtained withsubstantially no. harmonics of the signalling frequency, and a devicetuned to a frequency outside the range of frequency to be received, thearrangement of said impedance device and said tuned circuit being suchthat when no signals are being received the amplifying device oscillatesat a frequency determined by said tuned circuit and that when signalsare being received a current fed back over said path causes the strengthof weak and strong signals in the input circuit to approximate to thesame strength inthe output circuit.

6. An amplifying circuit for receiving alternating current signals,comprising an amplifymg device having an input and an output cir-' cuit,a path from said output to said input circuit including a non-linearimpedance device whose resistance varies with the strength of signals insaidoutput circuit, and a device tuned to a frequency outside the rangeof frequencies to be received, the arrangement of said non-linearimpedance device and said tuned device being such that when no signalsare being received the amplifying device oscillates at a frequencydetermined by said tuned device and that when signals are being receivedthe current fed back '1. A receiving circuit for alternating currentsignals of a predetermined frequency, comprisins an input circuit, asignal responding device, a tuning arrangement between said inputcircuit and said signal responding device by which currents ofpredetermined frequency are alone enabled to reach said signalresponding device, and an amplifier having a feed back circuit to bringit to a state of oscillation and including a variable impedance devicehaving a time factor which causes the amplitudes of the weaker andstronger signals to more nearly approach each other without causing theappreciable generation of harmonic frequencies located between saidinput circuit and said tuning arrangement. 8. A receiving circuit foralternating current signals of a predetermined frequency which may bemixed with currents of other frequencies, comprising an input circuit, asignal responding device responsive only to signals of predeterminedfrequency, a device responsive to other frequencies arranged to reducethe sensitivity of said signal responding device, and an amplifiernormally in a state of oscillation and including in its feed backcircuit a device for reducing the variation of amplitude of all currents"received over said input terminals which responds to high frequencyharmonic less quickly than to the predetermined frequency and therebyensures that the sensitivity reducing device is substantially notaffected by harmonics of signalling frequency derived from saidamplitude variation reducing device. 9. An amplifying circuit forreceiving alternating current signals, comprising an amplifying devicehaving an input and an output circuit, a path from said outputcircuit'to said input circuit including an impedance device in the formof bridge circuit variably controlled by current in the output circuit,and a device tuned to a frequency outside the range of frequencies to bereceived, the arrangement of said impedance device andsaid tuned circuitbeing such that when no signals are being received the amplifying deviceoscillates at a frequency determined by said tuned .device and that whensignals are being received the current fed back over said path causesthe oscillations to cease and the amplification of the stronger signalsto be reduced as compared with the amplification of the weaker signals.

10. An amplifying circuit for receiving alternating current signals,comprising an amplifying device having an input and an output circuit, apath from said output to said input circuit including a bridge circuithaving a non-linear impedance device in one limb whose resistance varieswith the strength of signals in said ouput circuit applied to oppositeterminals of said bridge, and a device tuned to a frequency outside therange of frequencies to be received and connected to the other terminalsof said bridge, the arrangement of said non-linear impedance device andsaid tuned device being such that when no signals are being received theamplifying device oscillates at a frequency determined by said tuneddevice and that when signals are being received the current fed backover said path causes the oscillations to cease and the amplification ofthe stronger signals to be reduced as compared with the amplification ofthe weaker signals.

11. An amplifying circuit for receiving alternating current signals,comprising an amplifying device having an input and an output circuit, apath from said output circuit to said input circuit, including animpedance device comprising a resistance wire in a vacuum or gasfilledenvelope variably controlled by current in the output circuit, and adevice tuned to a frequency outside the range of frequencies to bereceived, the arrangement of said impedance device and said tunedcircuit being such that when no signals are beingreceived the amplifyingdecauses the amplification of the stronger signals to bereduced ascompared with the amplification of the weaker signals.

